The present invention relates to an oscillating type compressor which can be advantageously used for compressing a fluid such as air.
Generally, an oscillating type compressor comprises: a crank case; a crankshaft rotatably supported by the crank case; a cylinder provided in the crank case; a cylinder head mounted on a distal end portion of the cylinder; an oscillating type piston connected to the crankshaft and capable of reciprocally moving within the cylinder while being oscillated, the piston defining a compression chamber in the cylinder; and a lip ring for providing a seal between the piston and the cylinder.
The lip ring is made of, for example, a resin material having self-lubricating properties. It comprises an annular attachment portion located in a radially inner position and attached to an outer circumferential portion of the piston, and a lip portion which is bent from a radially outer portion of the attachment portion and extends towards the compression chamber, and adapted to make slidable contact with an inner circumferential surface of the cylinder when the compression chamber is compressed. The piston comprises a disc portion disposed on a side of the crankshaft and a retainer disposed in a face-to-face relationship to the disc portion on a side opposite to the crankshaft. The attachment portion of the lip ring is held by the disc portion and the retainer, to thereby fix the lip ring to the piston (for example, see Japanese Patent Public Disclosure No. 9-144666).
In a conventional oscillating type compressor such as that mentioned above, when the piston reciprocates within the cylinder while being oscillated, only two portions of the entire periphery of the lip portion of the lip ring, which portions are located in a direction of oscillation of the piston, are subject to wear by being strongly pressed against the inner circumferential surface of the cylinder. Therefore, a problem of premature fracture of the lip ring occurs, due to wear of the above-mentioned two portions, although the remaining portion of the lip ring is worn by only about 30% of its original thickness. The lip ring is fixedly connected to the disc portion of the piston so as to prevent leakage of air from the compression chamber. Therefore, when the oscillating type compressor is operated, concentrated wear occurs at two portions of the lip portion located in a direction of oscillation of the piston, so that the life of the lip ring becomes short.